The H5N1 bird flu virus replicates far more aggressively in people than common human flu viruses, a study of patients in Vietnam has found, offering further insight as to why the virus is so deadly.

The study, published online in the journal Nature Medicine, also found that the virus gets into the blood stream of many of the humans it kills, which means the virus can spread to other parts of the body.

Dr Menno de Jong, of the Oxford University Clinical Research Unit at Vietnam's Hospital for Tropical Diseases in Ho Chi Minh City, says that the unusually high viral loads trigger intense cytokine responses, an immune system overreaction that can be fatal.

Cytokines are proteins in the immune system that fight off intruders such as bacteria and viruses.

"During H5N1 infection, the [cytokine] response seems to be very, very intense. Cytokines want to get rid of this intruder but if you have very high levels of cytokines, it can also damage the body ... it can be directed against your own cells and organs," says de Jong.

The study involved 18 people infected with H5N1 and eight with human flu in 2004 and 2005 in Vietnam.

Scientists found far higher viral loads in the nose and throats of those infected with bird flu than human flu.

Thirteen of those infected with H5N1 died and the virus was found in the blood of at least nine of them, suggesting it could have been transported out of the respiratory tract.

The virus was also found in the rectums of most of those with H5N1, suggesting it could have spread through the blood stream into the gastrointestinal tract.

Those with common flu had no virus in their blood or rectum. No one died in that group.

"The fatal outcome of H5N1 infections seems to be associated with high levels of replication of the virus and also the detection of the virus in the blood," says de Jong.

The team was able to draw a connection between those who were most ill and the level of cytokines found in them.

"We found that levels of cytokines were much higher in H5N1 patients than in the human flu cases. Again, the highest levels of cytokines were found in those who died of H5N1," he says.

"The high levels of the virus triggered an overwhelming inflammatory response that contributed to lung dysfunction and eventual death."

De Jong highlights the need to stop the virus replicating.

"What's important is to stop the replication as soon as possible, so you prevent damage to the lungs and prevent the inflammatory response to the virus," he says.

But he says that early diagnosis is a challenge, especially in remote places where health services are not readily available.